International
Tables for
Crystallography
Volume C
Mathematical, physical and chemical tables
Edited by E. Prince

© International Union of Crystallography 2006
International Tables for Crystallography (2006). Vol. C. ch. 4.2, p. 215

Section 4.2.3.2.2. Specimen selection

D. C. Creaghb

4.2.3.2.2. Specimen selection

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Although the most important component in the experiment is the specimen itself, examination of the data files held at the US National Institute of Standards and Technology (Gerstenberg & Hubbell, 1982[link]; Saloman & Hubbell, 1986[link]; Hubbell, Gerstenberg & Saloman, 1986[link]) has shown that, in general, insufficient care has been taken in the past to select an experimental device with characteristics that are appropriate to the specimen chosen. Nor has sufficient care been taken in the determination of the dimensions, homogeneity, and defect structure of the specimens. To achieve the best results, the following procedures should be followed.

  • (i) The dimensions of the specimen should be determined using at least two different techniques, and sample thicknesses should be chosen such that the Nordfors (1960[link]) criterion, later confirmed by Sears (1983[link]), that the condition [2\le\ln(I_0/I)\le4\eqno (4.2.3.7)]be satisfied. This enables the best compromise between achieving good counting statistics and avoiding multiple photon scattering within the sample.

    Wherever possible, different sample thicknesses should be chosen to enable a test of equation (4.2.3.1)[link] to be made. If deviations from equation (4.2.3.1)[link] exist, either the sample material or the experimental configuration, or both, are not appropriate for the measurement of [\mu_l]. If the attenuation of the material under test falls outside the limits set by the Nordfors criterion and the material is in the form of a powder, the mixing of this powder with one with low attenuation and no absorption edge in the region of interest can be used to bring the total attenuation of the sample within the Nordfors range.

  • (ii) The sample should be examined by as many means as possible to ascertain its regularity, homogeneity, defect structure, and, especially for very thin specimens, freedom from pinholes and cracks. Where a diluent has been used to reduce the attenuation so that the Nordfors criterion is satisfied, care must be taken to ensure intimate mixing of the two materials and the absence of voids.

    Since the theory upon which equation (4.2.3.1)[link] is based envisages that each atom scatters as an individual, it is necessary to be aware of whether such cooperative effects as Laue–Bragg scattering (which may become significant in single-crystal specimens) and small-angle X-ray scattering (SAXS) (which may occur if a distribution of small voids or inclusions exists) occur in polycrystalline and amorphous specimens. Knowledge that cooperative scattering may occur influences the choice of collimation of the beam.

  • (iii) The sample should be mounted normal to the beam.

References

First citation Gerstenberg, H. & Hubbell, J. H. (1982). Comparison of experimental with theoretical photon attenuation cross sections between 10 eV and 100 GeV. Nuclear data for science and technology, edited by K. H. Bockhoff, pp. 1007–1009. Amsterdam: North-Holland.Google Scholar
 First citation Hubbell, J. H., Gerstenberg, H. M. & Saloman, E. B. (1986). Bibliography of photon total cross section (attenuation coefficient) measurements 10 eV to 13.5 GeV. Report NBSIR 86-3461. National Institute of Standards and Technology, Gaithersburg, MD, USA.Google Scholar
 First citation Nordfors, B. (1960). The statistical error in X-ray absorption measurements. Ark. Fys. 18, 37–47.Google Scholar
 First citation Saloman, E. B. & Hubbell, J. H. (1986). X-ray attenuation coefficients (total cross sections): comparison of the experimental data base with the recommended values of Henke and the theoretical values of Scofield for energies between 0.1–100 keV. Report NBSIR 86-3431. National Institute of Standards and Technology, Gaithersburg, MD, USA.Google Scholar
 First citation Sears, V. F. (1983). Optimum sample thickness for total cross section measurements. Nucl. Instrum. Methods, 213, 561–562.Google Scholar








































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